The employment of thermoplastic elastomers as solid rocket propellant binders required a deviation from conventional processing techniques before they became a reality. The earlier approaches to employing thermoplastic elastomers as propellant binders had been centered around using standard state-of-the-art processing techniques; however, artisans, after much experimental work, concluded that it would be impractical, if not impossible, to mix solid particulates at the levels of interest into most thermoplastic elastomers while they are held above their melting points.
After a new conception and a subsequent reduction to practice, the desires of the propellant industry became a reality after being provided the combinations of the conceived process to prepare melt-formable composite rocket propellants. The completed invention offers a new beginning in the propellant industry. The invention which was filed on June 12, 1981, as Patent Application Ser. No. 272,859 issued as U.S. Pat. No. 4,361,526 on Nov. 30, 1982 to Henry C. Allen, and it is assigned to The United States of America as represented by the Secretary of the Army, Washington, D.C.
The invention of thermoplastic composite rocket propellant provides processes for practical formulation and additionally provides processes for reclaiming the propellant ingredients from the finished grain when desired, and thus, in actuality, the invention provides new techniques for the solid propellant artisan to overcome obstacles of the conventional prior art processing techniques. Thus, the advantages for utilizing a wide range of thermoplastic elastomers as solid propellant binders were soon recognized.
The advantages of thermoplastic rocket propellants are described in U.S. Pat. No. 4,361,526, and a practical formulation and process for preparation thereof are taught therein. However, it has become apparent through interpretation of stress versus strain data taken on propellant of the aforementioned formulation that the maximum potential with respect to mechanical properties for this type of propellant had not been achieved because of poor bonding of the thermoplastic binder to the oxidizer particles.
It is well known in the art that many propellant binders adhere poorly to the most commonly used composite propellant oxidizer, ammonium perchlorate (AP), and consequently it is common practice in the art to include in certain composite propellants small quantities of materials known as bonding agents to enhance the adhesive bond between the binder and AP. More detailed teaching on the use of bonding agents in composite propellants is to be found in the patent literature, for example in U.S. Pat. Nos. 3,745,074 and 3,762,972. It was because of this prior art teaching that the above mentioned formulation included as an intended bonding agent HX752, a well-known bonding agent for propellants having hydroxyterminated polybutadiene (HTPB) as binder. It has now been found that HX752 as well as other conventional bonding agents for chemically cured composite propellants have only minimal effect on the mechanical properties of thermoplastic composite propellants. Such conventional bonding agents depend on the agent's ability to form an adhesive coat on the oxidizer particles on the one hand and to chemically bond to the binder polymer during the propellant cure process on the other. Thermoplastic elastomers of the styrene-diene block copolymer type do not contain sites which are chemically reactive with conventional bonding agents, and thus no chemical bonding between binder and bonding agent can occur.
Conventional bonding agents for chemically cured AP-oxidized composite propellants do not form chemical bonds with the AP particles. This is true of even those nominal bonding agents which contain amine groups in their structures which chemically react with the AP particles to form amine perchlorates and liberate ammonia according to the following reaction: ##STR1## where R.sub.1, R.sub.2 and R.sub.3 comprise a combination of hydrogen and various organic groups such as have been found useful in certain bonding agents. The amine perchlorates, which are the true bonding agents in this illustration, are formed in situ on the AP particle surfaces and adhere to those surfaces through electrical rather than chemical forces. Sites for chemical reaction with the binder are usually derived from one or more of the groups R.sub.1, R.sub.2 and R.sub.3. The main point is that the nominal bonding agent is not chemically bound to the AP particles, but rather that it consumes a part of the AP via chemical reaction to form the true bonding agent, a separate chemical entity that merely adheres to the underlying AP surface through electrical attraction. This has been shown by Ducote and Allen in U.S. Pat. No. 4,491,741 wherein the perchlorate salts of the amine-type nominal bonding agents known in the art as TEPAN and TEPANOL were previously prepared by chemical reaction apart from the propellant mix and then added to the mix as bonding agents, the results being that propellant mechanical properties were indistinguishable from those in which the nominal bonding agents were added to the mix. Other bonding agents likewise adhere to AP particles by non-chemical bonding forces since they do not chemically react with AP. Thus it is shown that strong adhesion to AP particles is possible without chemical bonding thereto, and indeed that essentially all bonding agents for AP-oxidized propellant adhere to AP without chemical bonding thereto.
We now examine the matter of bonding between a bonding agent and a propellant binder. Since it is quite easy to synthesize bonding agents with chemical groups which can participate in propellant cure reactions, and since such reactions produce a very strong chemical bond between propellant binder and bonding agent, there has been no incentive to discover bonding agents for chemically cured propellants which do not chemically react with the propellant binder. However, incidental to other studies, it has been found and is well known in the art that propellant binders sometimes bond strongly to various types of solid particles with which there is no chemical reaction. Examples may be found in the bonding of hydrocarbon propellant binders to aluminum, aluminum oxide, carbon and various other types of particles. Thus chemical bonding between a polymer (propellant binder) and a substrate (particle) surface is not a requirement for a strong bond provided that the substrate surface has suitable characteristics relative to the bonding requirements of the polymer, as is well documented in the classicial adhesion literature and is mentioned here only to complete the argument summarized in the following paragraph.
Since chemical bonding to AP particles does not occur in conventional AP-oxidized propellants, and since chemical bonding is not required for a propellant binder to bond strongly to certain types of particle surfaces, it should be possible to modify the surfaces of AP particles so that chemically unreactive thermoplastic elastomers will bond strongly thereto and thus greatly improve the mechanical properties of the resulting propellant. Such modification may consist of the deposition onto the AP particle surfaces of a thin film of a material which will both adhere strongly to the AP surfaces and provide the substrate characteristics to which thermoplastic elastomers of the styrene-diene block copolymer type will adhere strongly. This invention accomplishes such a modification.
Therefore, it is an object of this invention to provide a class of bonding agents for thermoplastic composite propellants which does not require chemical reaction with the binder in order to be highly effective in improving the mechanical properties thereof. It will be shown in the discussion below that this object is theoretically attainable, and data will be presented to show that the object has been attained in practice.
A further object of this invention is to provide the teaching directed toward the theoretical basis for a proposed mechanism of how the improved mechanical properties are achieved for thermoplastic composite propellants by employing a new class of bonding agents.
Still a further object of this invention is to provide formulations employing a new class of bonding agent to illustrate in a reduction to practice the improved mechanical properties resulting from the use of a new class of bonding agent for thermoplastic composite propellants.